Standard diaphragm pressure sensors for measuring hydrostatic (uniform) pressures are known which produce an output electric signal having an amplitude proportional to the pressure on the diaphragm. Although such pressure sensors require only one sensor element for sensing diaphragm deflections, it is a common practice to connect four sensors in a Wheatstone bridge configuration to maximize the output signal, the four sensors thereby constituting a single pressure sensor in application. Examples of such pressure sensors are U.S. Pat. Nos. 4,702,113 and 4,712,430 both to Wareham. Another pressure sensor, U.S. Pat. No. 4,770,045 to Nakagawa, has a plurality of strain gauges disposed on a diaphragm for high-sensitivity measurement of uniform pressures.
In silicon-based diaphragm pressure sensors, the bending stress on the diaphragm is typically sensed using a strain sensitive resistor which is diffused into a thin silicon diaphragm which is supported by a thicker silicon frame. A hydraulic pressure applied to one side of the diaphragm causes the diaphragm to bend which produces a stress in the resistor which may be electrically sensed. Such a resistor is referred to as a piezoresistor. In impurity-doped silicon sensors, the sensitivity of such a piezoresistor to stress is dependent upon the concentration of the diffused impurity, the direction of the current flow relative to the crystal lattice and the direction of the applied stress. Typically, p-type diffused resistors are oriented along the &lt;110&gt; directions in (100) silicon crystal. In this case, stress applied parallel to the resistor causes the resistance to increase and stress applied perpendicular to the resistor causes a decrease in resistance. Again, typically, four piezoresistors are connected in a Wheatstone bridge configuration to effectively constitute one pressure sensor.
The diaphragms typically utilized can be square, rectangular, or round. The rectangular diaphragm has been used in several different designs of pressure sensors for the measurement of uniform pressures such as that produced by liquids. In the rectangular diaphragm, the sensing element resistors are typically placed at or near the fixed edges and at or near the center of the diaphragm and connected in a Wheatstone bridge configuration. Depending on the aspect ratio (ratio of length to width of a rectangle) of the diaphragm, a problem with a relatively square configuration has been that stiffening of the diaphragm near its fixed edges can influence measurements of pressure in the sensing region of the diaphragm. A rectangular diaphragm having an aspect ratio of at least 2.3 to 1 achieves measurements in the sensing area of the diaphragm which are only minimally affected by the ends of the diaphragm.
Non-uniform pressures produced, for instance, by semi-rigid, non-fluid media present a problem for conventional diaphragm-type sensors since the pressure may not be uniform over the entire area of the diaphragm. In such a case, the sensing element resistors would be measuring a locally applied force plus strain on the diaphragm produced by applied forces in other regions of the diaphragm, resulting in inaccurate measurements. One solution to this problem is the deployment of an array of diaphragms each having its own separate sensing element. C. S. Weaver, et al., Interim Report to the National Heart and Lung Institute (Grant HL17604-01A1) 1976. However, because of the physical requirements of each diaphragm (such as the fixed edges and coupled circuitry) such a system has disadvantages in terms of the separations required between adjacent diaphragms, thereby decreasing resolution. An example of a non-uniform pressure requiring accurate, high resolution measurements is the transcutaneous measurement of blood pressure by the method of tonometry.